Corrugated paperboard is widely used as a material for constructing containers and other packaging applications. Untreated corrugated paperboard is strong, relatively inexpensive and may be recycled by composting or re-pulping. Conventional corrugated paperboard is constructed of two opposing liners and an intervening fluted medium secured between each liner with an adhesive, typically starch. The corrugated paperboard is heated to dry or cure the adhesive during its assembly on the corrugator.
Petroleum derived paraffin waxes and synthetic polymers have been used as coatings for many years in the manufacture of paper and paperboard as moisture retardants, water repellents, oil repellents, stiffeners, strengtheners, and release agents. Besides paraffin waxes, the material used most often in such products is polyethylene. However, other widely used polymers in the field include polymerized acrylics, vinyls, styrenes, ethylenes and copolymers or hetero-polymers of these monomers. The paper and paperboard to which these traditional materials are applied is difficult and often impossible recycle in standard paper mill processes because the petroleum derived polymers and, particularly, the petroleum waxes are non-biodegradable in mill white waters (circulated process waters) and discharge effluents. Furthermore, the residue of the petroleum waxes that is not removed from pulp fibers during the recycling processes causes severe operation and maintenance problems due to buildup that occurs on the screens and felts used during the process of forming and making the paper or paperboard sheet. Likewise, coatings and impregnated products made from or based on paraffin waxes and/or similar petroleum derivatives can only be re-pulped for recycling in specially configured re-pulping equipment that removes and separates the paraffin waxes. In addition, paper and paperboard coated or impregnated with petroleum waxes resist biodegradation and composting when disposed of in landfills and other waste disposal systems. Moreover, boxes containing petroleum waxes must be separated and deposited in separate landfill areas.
Paper and paperboard coated or impregnated with traditional synthetic polymers and hetero-polymers are also difficult to impossible to recycle owing to their resistance to separation from the fiber in the standard re-pulping processes resulting in significant fiber losses during efforts to re-pulp and recycle them. These coated or impregnated paper and corrugated board are also non-biodegradable and therefore resist composting.
As previously noted, water repellent packaging currently being used utilizes petroleum based liquid polymers or polymer film laminates (including polyethylene or similar film laminates such as polyolefin, polyester, polyvinyl alcohol, polyvinyl acetate, polystyrene, polypropylene, and the like) which are recyclable after extensive treatment, All of these laminates require the installation of specialized machinery that is far more expensive in terms of operating costs and/or recycled pulp fiber yields. The action of separating the fiber from the film damages some fibers causing them to be selected out of the recycled pulp while the separated film waste carries other fibers out of the re-pulpate.
The poultry box industry currently uses a wax to coat the boxes (EVA—poly (ethylene vinyl acetate) which takes a significantly long period of time to compost and leaves a chemical residue from those compounds. This process is known in the industry as curtain coating. There appears to be little or no decomposition of higher molecular weight resin and EVA fractions. Wax coated boxes are not considered to be biodegradable and do not meet the ecology friendly standards of many companies and are therefore not used by these companies.
The use of tallow to treat paper is well known in the prior art and a number of patents have attempted to use tallow to address the above noted problems. U.S. Pat. No. 2,840,138 issued Jun. 24, 1958 discloses the use of tallow fatty acids to impregnate and penetrate corrugated paper material to provide a wilt resistant material while U.S. Pat. No. 4,752,637 issued Jun. 21, 1988 is directed to a method of treating fiberboard to have moisture resistance with various mixtures of hydroxyl terminated esters such as tallow.
U.S. Pat. No. 6,103,308 issued Aug. 15, 2000 is directed toward a paper and paperboard coating composition using vegetable oil triglyceride as a paper coating while U.S. Pat. No. 6,201,053 issued Mar. 13, 2001 is directed toward various triglycerides mixed with catalysts for use as a waterproofing agent on paper coating.
U.S. Pat. No. 6,846,573 issued Jan. 25, 2005 discloses the use of hydrogenated triglycerides having a melting point above 50° as a coating material for the surface of paper products to improve wet strength and moisture resistance of the paper while allowing the paper product to being re-pulpable.
U.S. Pat. No. 7,870,992 issued Jan. 18, 2011 discloses a cardboard container having an outer shell which includes a moisture resistant barrier. The container further includes at least one insulating member disposed within the outer shell having a cellulose-based substrate substantially encapsulated in a polymeric film.
The combination of impregnation of the poultry boxes medium and liner boards with hydrogenated triglyceride and covering the outer surface of the liner boards with a dimethyl or ethylene terephthalate (PET) coating is disclosed in U.S. Pat. No. 8,455,068 issued Jun. 4, 2013. While this coating method resulted in corrugated poultry boxes which provided moisture resistance and were recyclable and re-pulpable, the boxes were only able to obtain a 30 minute Cobb value of about 20-30 causing water absorption of the boxes and structural damage to the sides and corners as well as the fold lines when the boxes were stacked for storage and shipping.
U.S. Pat. No. 4,871,406 issued Oct. 3, 1989 discloses the manufacture of a corrugated paperboard having a layer of plastic film heat laminated to the single face liner of a corrugated board. The film is supplied from a film roll and reeved over an unwrinkling device to smooth it before it is placed on the liner face. The film used for the lamination was a thermal plastic high density polyethylene (HDPE) having a heat seal layer of ethylene acrylic acid (EAA) which allowed the film to be adhered to the liner face. The '406 patent also discloses additional films which can be used for lamination such as coextruded polypropylene (PP) film and EAA or a polyester film coated with EAA. Corrugated boxes using the combined film lamination were used to package meat and poultry as well as viscous materials such as soft drinks, syrups and household chemical products.
U.S. Pat. No. 7,195,804 issued Mar. 27, 2007 discloses the manufacture of a paperboard structure adhered to a backing structure. The backing structure includes an orientated polymer film such as biaxial oriented polyethylene terephthalate film and a reinforcing scrim heat bonded together through the use of a thermal bonding layer to form a film composite.
Manufacturers have been trying to replace wax coated boxes for a number of years due to the fact that wax coated boxes do not compost and are extremely difficult to recycle.
As previously noted, the wax coated boxes are produced by a process known as curtain coating in which liquid wax is applied by pouring a curtain of wax over the exterior surface and/or interior surface of the box blank to form a uniform wax coating on the box blank.
The application of laminates to corrugated material has been undertaken for a number of years to try to solve the above noted problems with mixed success but have been unable to match the 30 minute Cobb values of wax-coated boxes which runs between 0 and 5. Furthermore, most of the lamination was done offline rather than inline which is an expensive manufacturing technique as well as incurring significant transportation costs to transport and store the offline laminated board for use on an inline corrugator.
One significant problem with non-wax coated boxes of corrugated paperboard when used for packaging of fresh poultry, seafood, meat and the like is that such boxes have a higher Cobb value over 20, typically between 40 and 80, and become wet from condensation of moisture within the box or leakage that occurs from the food product by fluid discharge from the product or melting of ice packed products. The corrugated box end corners, sides and fold lines become wet, resulting in a deterioration of box strength, box stacking capability and box tear strength leading to box crushing and tearing of the corners and sides of the boxes when the boxes are stacked.
Other problems with corrugated boxes used for fresh product shipping is that moisture from poultry, seafood and meat or other contained food product causes significant box strength weakening and provides box areas for bacterial growth and possible food contamination.
There thus exists a need for a compostable or recyclable moisture resistant corrugated food box which does not use wax or other non-biodegradable coatings. This problem has been substantially unresolved by the paper corrugating industry for the last decade.